The present invention relates to a picture display apparatus for displaying high quality television pictures or the like.
While, in the conventional television system, the horizontal synchronizing signal comprises a negative-polarity pulse which is inserted on the black level side of a picture signal, studio standards for a high quality television system of Japan specify that the synchronizing signal comprises a combination of two pulses of negative and positive polarities. The horizontal synchronizing signal comprising two pulses of opposite polarities is generally called a tri-state binary synchronizing signal.
The present invention is intended to ensure the positive restoration of this so-called tri-state binary synchronizing signal.
FIG. 3 is a block diagram of a conventional sync separation circuit and FIG. 4 shows a plurality of voltage waveforms at various parts of the conventional sync separation circuit. The conventional technique will now be explained with reference to FIGS. 3 and 4. In FIG. 3, numeral 11 designates a current amplifier circuit for a picture signal including tri-state binary synchronizing signals ((a) of FIG. 4), and 12 a peak clamp circuit for clamping the negative peaks of an output from the current amplifier circuit 11 at a given DC voltage level as shown in (b) of FIG. 4. In the peak clamp circuit 12, current flows during the time of each negative peak in the input so that the peak of an output signal is controlled in accordance with the current value and the peak is maintained at a constant level. Numeral 13 designates a monostable multivibrator which is triggered by the peak clamped output to generate a pulse voltage signal ((c) of FIG. 4), and 14 a gate circuit for utilizing the pulse voltage signal of the monostable multivibrator 13 to extract the output of the peak clamp circuit 12 during the pulse duration time. With this construction, a positive-polarity synchronizing signal is generated at the output of the gate circuit 14 as shown in (d) of FIG. 4.
The conventional technique has two disadvantages. The first disadvantage is that if a large noise is introduced into a composite signal including tri-state binary synchronizing signals and a picture signal, this noise triggers the monostable multivibrator so that the positive-polarity pulse width of a gate output cannot be restored properly. The second disadvantage is that a discrimination circuit having a given threshold value must be newly added to determine the time of rise from the zero potential of the positive-polarity pulse of the tri-state binary synchronizing signal and that the restoration of a perfect tri-state binary synchronizing signal cannot be effected by the above-described construction of the conventional circuit.